Passive optical components (for example, lenses, prisms, and mirrors) are well-known and are used in optical systems to collect, distribute, or modify optical radiation. A desire for smaller and smaller optical devices has led to the development of technologies for fabricating miniature optical components, and refractive microlenses with diameters of two millimeters down to a few microns can now be fabricated with high quality.
Such microlenses, as well as microlens arrays, have been made by a number of different techniques, each having a different set of strengths and weaknesses. Techniques that have been used for the preparation of spherical lens arrays include photopolymer reflow, gray scale lithography, laser ablation, dip coating of curable monomers on patterned hydrophobic/hydrophilic substrates, ink jet printing of curable monomers, diamond turning, ion beam or wet etching, and electrodeposition.
Optical requirements can vary greatly from one application to another, however, and, for some applications, spherical aberrations can come into play. The use of aspherical microlenses can reduce such spherical aberrations. Thus, aspherical microlenses can be a desirable alternative to spherical microlenses in some cases.
The techniques that can be used to fabricate high numerical aperture, aspheric microlens arrays are considerably fewer in number, however, than those that can be used to produce spherical microlens arrays. Such techniques include primarily gray scale lithography, which, although widely used for the fabrication of microlens arrays, is generally somewhat limiting as to the microlens height or depth of field that can be achieved.